The most common strategies for the treatment of wastes with potential heavy metal leaching problems can be placed in one of two categories: 1) chemical stabilization or fixation, which includes the treatment of the residue with a chemical additive in such a manner so that the contaminant of interest is converted to its least soluble form, and 2) solidification, which includes the addition of a binder, cement or pozzolan and lime to the residue to produce a matrix of low permeability that retards or reduces the rate of contaminant migration into the surrounding environment. A third, less common, method includes the process of washing the contaminated waste in order to dissolve the metal contaminants contained in the waste, and recapturing the metals from solution in subsequent precipitation or filtering steps.
Probably the most common form of treatment of heavy metal-bearing residues is chemical stabilization using lime or an alternative alkaline reagent to adjust the pH of the residue (or leachate from the residue) to a pH range that will promote the formation of insoluble metal-hydroxy complexes. Many polyvalent metals, however, do not form insoluble hydroxy compounds at elevated pH values and in fact exhibit amphoteric properties. An amphoteric metal is a metal that exhibits high solubility at both a high and low pH and minimum solubility in a very narrow pH range in between. Lead is characteristic of this phenomenon. As a result, the addition of lime to chemically stabilize lead is generally not an effective treatment approach.
In attempts to remedy this problem and to provide for the stabilization of lead over a wider pH range, researchers have proposed the use of other additives. Some of these additives are described in subsequent descriptions of relevant patents.
As will become apparent to those familiar with the chemistry involved in stabilization reactions, many of the processes described in these patents makes use of individual elements that represent the cationic component of polyprotic acid oxyanions. This includes elements such as phosphorus, boron, sulfur and carbon. These elements can form acid oxyanions such as phosphates, borates and sulfates and carbonates in non-reducing aqueous environments. Each acid oxyanion is capable of combining with lead to produce relatively insoluble lead-oxyanion complexes. Each oxyanion also tends to perform best within a certain pH range.
It is of further interest that although polyprotic acid oxyanions can form insoluble lead complexes, these oxyanions will also react with multivalent noncarbonate hardness-producing elements, such as calcium and magnesium, that may be present in the waste material or may be introduced (e.g., calcium hydroxide) as a treatment additive. The term hardness as used herein is meant to refer to soluble divalent compounds and in particular calcium and magnesium. Hardness or hard water is a term that commonly describes waters that contain relatively high concentrations of calcium and magnesium. A soft water contains relatively low concentrations.
The presence of soluble calcium and magnesium (or other multivalent metal cations) will therefore interfere with the effectiveness of acid oxyanion treatment by reacting to produce insoluble calcium and magnesium acid oxyanions such as calcium or magnesium phosphates or sulfates. Due to the presence of these interfering elements, additional quantities of the treatment additive must be added to meet both the noncarbonate hardness demand and the heavy metal (i.e., lead) demand of the waste. It is noteworthy that calcium is commonly introduced in the form of quicklime or hydrated lime to many solid residues, thereby inhibiting the effectiveness of the aforementioned acid oxyanions.
Among prior art descriptions that are reviewed are the following:
______________________________________ References Cited U.S. PAT. DOCUMENTS ______________________________________ 4,049,462 9/1977 Cocozza 4,375,986 3/1983 Pichat B1 3,837,872 2/1986 Connor (Reexamination Certificate) 4,443,415 4/1984 Queneau et al. 4,645,651 2/1987 Hahn et al. 4,671,882 6/1987 Douglas 4,701,219 10/1987 Bonee 4,737,356 4/1988 O'Hara et al. 4,891,130 1/1990 Pitts 4,917,733 4/1990 Hansen 5,009,793 4/1991 Muller 5,037,479 8/1991 Stanforth 5,045,115 9/1991 Gmunder et al. 5,150,985 9/1992 Roesky et al. 5,193,936 3/1993 Pal et al 5,202,033 4/1993 Stanforth 5,230,876 7/1993 Hutter ______________________________________
In U.S. Pat. No. 4,737,356, O'Hara and Surgi describe a stabilization process in which soluble phosphate and lime is added to the residues from municipal waste combustors to control lead and cadmium solubility. Although the authors do not describe the specific mechanisms involved in the fixation process, the process as outlined makes use of phosphorous, which comprises the cationic portion of the phosphoric acid oxyanion, to form insoluble lead-phosphate complexes, and lime to control the pH in an elevated range and to complex cadmium into insoluble hydroxy-cadmium compounds (e.g., cadmium hydroxide). It is noteworthy that the inventors stress the need for the presence of a free lime source to achieve effective stabilization of lead and cadmium.
In U.S. Pat. No. 4,671,882 Douglas describes a similar process in which phosphorous and lime are added to hazardous sludges to form metal phosphates. The major difference between this process and the O'Hara el. al process appears to be the application of the former to the stabilization of municipal waste combustor residue and the latter to the addition of the lime and phosphorous as pan of wastewater treatment operations in which coagulants are also added to help precipitate the resulting sludges.
In U.S. Pat. No. 5,037,479, Stanforth describes a method for treating lead, cadmium and zinc in which phosphorus, in the form of phosphate salts or phosphoric acid, and boron, in the form of boric acid, is added to a waste product containing the aforementioned metals, along with buffering agents (e.g., magnesium oxides, magnesium hydroxides, calcium carbonate, and magnesium carbonate). Stanforth claims that such an approach provides stabilization of lead, cadmium and zinc when subjected to acidic leaching tests or distilled water tests. In effect, Stanforth's approach uses the proposed buffering agents to neutralize the acid, in acidic leaching tests, to maintain an alkaline pH condition where zinc and cadmium will form insoluble carbonates or hydroxides complexes, and where lead will combine with phosphates or borates, both of which are polyprotic acid oxyanions, to form insoluble lead-phosphate or lead-borate complexes. The basic approach is similar to the O'Hara and Douglas processes. It is noteworthy that Stanforth recommends the introduction of buffering agents, which include calcium and magnesium carbonates, to assist in controlling the pH of the waste product. There is no recognition by Stanforth of the interfering effects of calcium or magnesium on acid oxyanion lead stabilization.
In U.S. Pat. No. 5,193,936, Pal and Yost describe a process in which gypsum and phosphoric acid (or appropriate calcium, sulfur and phosphorus substitutes) are sequentially added to contaminated lead soils and mixed in the presence of moisture and permitted to cure to produce a matrix consisting, according to the authors, of insoluble lead sulfate and lead phosphate complexes. The inventors, in this case, are making use of phosphate and sulfate oxyanions to produce insoluble lead-phosphate and lead-sulfate complexes.
In U.S. Pat. No. 4,701,219, Bonee discloses a method for reducing the leaching of vanadium and nickel from carbon and metal sorbents and catalysts used in petroleum cracking processes, by using either lime, calcium fluoride or calcium hydroxide, or a mixture of these compounds, as treatment agents. In this process, Bonee is using calcium, present in all the proposed treatment additives, to form a relatively insoluble calcium vanadate complex and a relatively insoluble hydroxy-nickel complex.
The use of sodium carbonate is proposed by a number of researchers for the purpose of forming metal carbonate complexes, to assist in the adjustment of pH for stabilization control, or to assist in the recovery of specific metals from solution.
In U.S. Pat. No. 5,202,033, Stanforth describes a method of treating solid waste in soil containing unacceptable levels of leachable metals such as lead, cadmium, arsenic, zinc, copper and chromium, using a phosphate source, a carbonate source or ferrous sulfate. Stanforth emphasizes that the treatment is accomplished by adding materials containing phosphates or carbonates that can enter into solution to form insoluble metal phosphates or metal carbonate compounds. Where chromium is present, Stanforth recommends the use of ferrous sulfate as a treatment additive to reduce hexavalent chromium, which is highly toxic, to tetravalent chromium, which is less toxic. Stanforth also recommends the use of a pH controlling agent to assist in the immobilization process. To supply a phosphate source, Stanforth recommends the use individually or in combination, of a number of phosphate salts as well as phosphoric acid. To supply a carbonate source, Stanforth recommends the use of sodium carbonate, sodium bicarbonate or calcium carbonate. For pH control, Stanforth recommends the use of magnesium oxide, magnesium hydroxide, calcium oxide and calcium hydroxide. It is noteworthy that Stanforth in his recommendation for soil treatment, suggests using, as a carbonate source, one or more carbonate salts including sodium bicarbonate, sodium carbonate or calcium carbonate. Stanforth's intent in his recommendation is to supply a carbonate source for the sole purpose of promoting the formation of metal carbonate complexes and makes no distinction between sodium or calcium carbonate. Stanforth, in fact, recommends the introduction of calcium and magnesium in several additives. There is no recognition by Stanforth of the interfering effects of calcium or magnesium on lead oxyanion treatment.
In U.S. Pat. No. 4,443,415, Queneau discloses a method for recovering vanadium from a petroleum coke residue by slurrying the coke in an aqueous solution of sodium carbonate to form sodium vanadate and sodium sulfate, and aerobically digesting the slurry at elevated temperatures under pressure to digest the carbon and to generate a vanadate liquor for recovery. The process described makes use of sodium carbonate and a high temperature and pressure process for vanadium recovery but makes no reference to its use to assist in the stabilization of lead bearing waste products.
In U.S. Pat. No. 4,645,651, Hahn proposes an alternative method for recovering vanadium from vanadium-bearing residues by combining the residues with a superstoichiometric quantity of a mixture of sodium carbonate and sodium sulfate, heating the mixture to its melting point and using a sodium carbonate solution to leach out the vanadium into solution. Once again the process described makes use of sodium carbonate to assist in the extraction of vanadium into a vanadate solution for recovery, but no reference to the use of sodium carbonate or the vanadate solution for lead stabilization is provided.
In U.S. Pat. No. 4,891,130, Pitts describes a method for recovering vanadium from an aluminosilicate material such as kaolin clay by using sodium carbonate or potassium carbonate to extract the vanadium as soluble alkali vanadate, preferably at a temperature near the boiling point of the alkali carbonate solution.
As previously noted, solidification processes use encapsulating reagents (e.g., cements) to retard contaminant migration.
In U.S. Pat. No. 4,049,462, Cocozza describes a solidification process in which desulfurization residues are treated to form a hardened cement-like mass by the addition of an alkaline reagent such as cement-kiln dust, which is a fine powdery waste product derived from the manufacture of Portland cement, in the presence of sufficient water, pH adjustment to 7 and one to two weeks of air drying. Similarly in U.S. Pat. No. 4,917,733, Hansen describes a process in which cement-kiln dust is added to fly ash collected from baghouses or precipitators of municipal waste combustors with excess lime and leachate from landfills to produce a hardened mortar-like material.
In U.S. Patent Reexamination Certificate No. B 13,837,872, Connor describes a process in which an alkali-metal silicate is mixed with waste material and a silicate setting agent, from a group consisting of Portland cement, lime, gypsum and calcium chloride, to form a solidified silicate matrix within which the pollutants are entrapped.
In U.S. Pat. No. 4,375,986, Pichat describes a process in which waste material with a pH less than 2 is converted into a solid material using coal fly ash and a neutralizing agent such as lime or lime-containing materials or Portland cement. Pichat in his process emphasizes the economic advantages of using waste materials such as coal fly ash, which is pozzolanic, instead of Portland cement concrete or other solidifying reagents such as sodium silicates.
In U.S. Pat. No. 5,150,985, Roesky describes a process for treating low-lime content dusts collected from incinerator plants by mixing the dusts with cement and water, compacting the mixture into discrete shapes and hardening the mixture in an autoclave with saturated steam and a pressure of at least one bar.
As previously noted, washing processes have also been proposed for metal-bearing waste treatment.
In U.S. Pat. No. 5,009,793, Muller describes a process for separating heavy metals, including lead, from waste materials by treating the contaminated materials with mineral acids to dissolve the heavy metals as water soluble salts, followed by subsequent re-precipitation of the metals as hydroxides. In U.S. Pat. No. 5,045,115, Ginunder discloses a similar method for washing-out metals from the ash from combustion plants. The method involves a washing step to dissolve the metals, followed by subsequent treatment of the wash water to remove the metals from solution.
The process disclosed by the present invention does not rely on solidification or washing, as presented above, to mitigate leaching problems from lead contaminated wastes. It does not rely solely on pH control to buffer or adjust the pH of the waste to a narrow range where lead is insoluble. The process makes use of acid oxyanions to stabilize the lead in these waste materials, and alkali-metal carbonates to increase the efficiency of the lead stabilization process by reacting with elements (e.g., calcium and magnesium) that can preferentially react with acid oxyanions and thereby reduce their effectiveness as treatment reagents for heavy metals, such as lead.
None of the previous inventors, in their disclosures, have recognized the effectiveness of polyprotic acid oxyanions as a family of compounds that could stabilize lead-bearing waste materials. They have not recognized the interfering effects of multivalent noncarbonate hardness-producing elements, such as calcium and magnesium, in the treatment of lead-bearing wastes, nor have they recognized or identified the advantages of using alkali-metal carbonates to mitigate these effects.
It is noteworthy that elements comprising the cationic portions of fully dissociated polyprotic acid oxyanions, including such elements as phosphorus, boron, vanadium, arsenic, selenium, inorganic carbon, sulfur and chromium are present in many byproducts or waste materials, but are usually bound up in insoluble multivalent metal complexes, such as calcium and magnesium oxyanion complexes. It is possible to release these elements into solution by using alkali-metal carbonates to promote the formation of soluble sodium and potassium oxyanion salts and insoluble multivalent hardness-producing carbonates (e.g., calcium carbonate and magnesium carbonate). The use of sodium or potassium carbonate to extract vanadium from solids has been disclosed by others for the purpose of recovering vanadium in the form of sodium vanadate, but no prior inventions have disclosed the method of using titis approach to extract reagents for the purpose of lead stabilization. The release of these elements into solution to react with lead, to form insoluble lead oxyanion complexes, could make use of these byproducts or waste materials and avoid the need to add virgin sources of the aforementioned elements to stabilize waste materials. This could result in more favorable waste stabilization management and economics.
It is of further interest that many combustion or incineration processes (e.g., municipal solid waste or medical waste incineration) release acid gas (e.g., HCl or SO.sub.2) that must be neutralized prior to release into the atmosphere to comply with air pollution emission regulations. The principal product used for acid gas control is quicklime or hydrated lime which is injected into the combustion gas stream. To achieve satisfactory efficiencies, excess lime (greater than stoichiometric acid requirements) is typically added in this process. Excess lime injected into the combustion gas stream of these processes is normally recaptured in baghouses or fabric filters and subsequently combined with the incineration residues or ash. In some processes this excess lime is used as an alkaline reagent to chemically stabilize the combustion residues. For example, in many municipal solid waste incinerators, in addition to its use in neutralizing acid gas emissions, excess lime is also injected into the combustion gas stream or air pollution control systems to assist in chemically stabilizing the cadmium in the incinerator ash so that cadmium leaching from the ash will not exceed allowable regulatory leaching levels. As previously outlined, this excess lime, which is helpful in stabilizing elements such as cadmium or zinc is not helpful and, in fact, can be detrimental to lead stabilization. It is also noteworthy that sodium carbonate or trona ore, from which sodium carbonate is derived, could be used as a substitute or in addition to lime in the neutralization of acid gas. The introduction of sodium carbonate or trona for acid gas treatment in concert with a polyprotic acid oxyanion could be used as an alternative method for both acid gas control and lead stabilization.